It is known to fabricate structural elements from resin/filament composite systems in which the filaments provide the tensile strength and the resin serves as a matrix to maintain the filaments in alignment. The filament material is often in the form of a fabric woven from tow, relatively loose strands of essentially untwisted filaments. The filaments may be glass, high modulus graphite, or other material having desired characteristics. The fabric may then be impregnated with a suitable resin such as an epoxy, polyester or polyimide resin to form what is known as prepreg material which, generally speaking, comprises flat sheets of fabric impregnated with uncured resin. Layers of prepreg are typically laminated and cured at high temperature and pressure to form the desired article.
The proportion of resin to filament is dictated by the strength-to-weight requirements of the fabricated parts. In particular, since the tensile strength comes from the filaments rather than the resin, a low resin content is desirable. While the proportions will vary according to the materials and the application, a cured prepreg sheet comprising a woven graphite fabric (made of high modulus carbon filaments) impregnated with a 350.degree. F. curing epoxy resin should contain approximately 35% resin by weight.
However, the lay-up procedure for molding the prepreg sheets sometimes requires placement of one or more sheets against a non-horizontal surface, as for example a vertical mold surface. It is thus necessary that the prepreg be sufficiently tacky to stick to such a surface during the lay-up. As a practical matter it is, therefore, necessary that some uncured resin is present on the faces of the prepreg. It has been found that a prepreg containing the desired end proportion of resin is insufficiently tacky to be suitable for lay-up against non-horizontal surfaces because substantially all resin is absorbed by the fabric.
The common solution to the problem has been to impregnate the fabric with a resin content in excess of that required in the cured article (e.g. 42% rather than 35%) and to bleed the excess resin off during the curing process. This is accomplished by laying porous cloth such as fiberglass cloth over the assembled prepreg sheets, but separated from them by a porous release film, so that the excess resin can be absorbed by the fiberglass bleeder which is later removed.
It is immediately evident that such a solution is wasteful, particularly when costly resin is used. Since the resin that is bled off is subjected to the same temperature and pressure as that remaining in the prepreg sheets, it cures, and cannot be recovered for subsequent use.
An additional difficulty with the use of so-called "excess resin" prepregs is that it may be difficult or impossible in certain applications to properly bleed off the excess. Moreover, the proper thickness and weight of the fiberglass bleeder is a variable function of the resin content and the prepreg fabric. Thus, even though the proper bleeder parameters can be established empirically, the use of bleeders represents an additional complexity and a potential source of failure.